CAMBBIA STEEL. 306 



EXPLANATIONS OF TABLES OF SAFE LOADS FOR BEAM BOX-GIRDERS AND 

 PLATE GIRDERS, PAGES 306 TO 326 INCLUSIVE. 



For cases in which the loads to be carried exceed the capacities of single rolled 

 beams or ordinary beam girders composed of two or more beams with the usual 

 bolts and separators, it is necessary to use built-up sections. 



BEAM BOX-GIRDERS. A useful and economical section of this kind can be com- 

 posed of two rolled beams with plates riveted to the top and bottom flanges, making 

 a beam box-girder, tor which tables of safe uniformly distributed loads are given 

 on pages 306 to 316 inclusive. 



The safe loads given in the tables include the weights of the beam box-girders, 

 and are figured from the moment of inertia or the section modulus after making the 

 necessary deductions for rivet holes, the fibre stress used in the calculations being 

 15 000 pounds per square inch of net section. 



Beam box-girders are particularly useful for supporting wide walls and in other 

 locations up to the limits of their capacity, but they should not be placed where ex- 

 posed to moisture, as the section is such that access cannot be had to their interior 

 for inspection and painting. 



PLATE GIRDERS. In cases where the widths of beam box-girders would prohibit 

 their use, and for loads greater than their capacities, plate girders composed of plates 

 and angles may be used. 



Tables of safe loads uniformly distributed for plate girders from 24" to 48" deep 

 are given on pages 317 to 326 inclusive. 



The loads given in the tables include the weights of the girders and are calculated 

 from the moment of inertia or the section modulus after making a proper deduction 

 for rivet holes, the fibre stress used in the calculation being 15 000 pounds per square 

 inch of net section. 



Although the tables do not show the stiffener angles for plate girders, care ihould 

 be taken that these are provided in all cases where necessary to prevent buckling of 

 the web due to the shearing action therein. The stiffeners should be made of angles 

 riveted to the web. fitted tightly between the top and bottom flange angles, and they 

 should be provided, at the end of the girders, of such size and number as to be 

 capable of carrying the total reaction at each end to the supports. Stiffeners should 

 also be provided at intervals along the girder, spaced at suitable distances apart, as 

 determined by the formula and explanations on pages 94 and 95. 



Care should also be taken in arranging the rivet spacing for connecting the flange 

 angles to the web, so that sufficient rivets are provided to properly transmit the 

 stresses which act between these two portions of the construction. This will require 

 the rivets to be spaced more closely a*he ends than at the center, and the exact 

 spacing at any point along the girder may be obtained by dividing the product of 

 the distance between the center lines of the rivet holes in the two flanges and the 

 resistance of one rivet by the total vertical shear at the given point, thus : 



p = -^-in which 



S = the total vertical shear, in pounds, at the point under consideration. 



r = the resistance of one rivet, . ., the bearing value or shearing value, whichever 



is the smaller, expressed in pounds, 

 h = the depth of the girder between the upper and lower center lines of rivets, 



expressed in inches. 

 p = pitch of rivets in the flange angles, expressed in inches. 



The formula above will give the theoretical rivet spacing at any point in the 

 flanges due to the total shear, but in practice the pitch for various portions of the 

 length should be stated for the least possible number of spacing panels containing 

 an even number of spaces, the pitch in each of which should preferably be expressed 

 in even inches or even inches and halves or quarters of an inch, and the usual limits 

 of pitch will vary from 2H" to 6". 



The rivet spacing should also conform to the rules given on page 358, and in 

 cases where loads are applied directly to the flanges, sufficient rivets must be pro- 

 vided to carry these in addition to the rivets necessary for securing the web and 

 flanges together as explained above. 



It should also be noted that the safe loads given in the tables are based on the* 

 assumption that the girder is supported laterally, otherwise a proper reduction in 

 the allowable safe load must be made, as explained in connection with beams on 

 pages 82 and 83. 



The weights of beam box-girders and plate girders in the tables are expressed in 

 pounds per lineal foot, including the rivets necessary to secure the web and flanges 

 together, but the weights do not include any allowance for brackets, stiffeners, con- 

 nections or other details, as these will vary, subject to the conditions of each case. 



